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It should be clear that you just need to downgrade the xserver with a ppa to use fgrlx 12-6-legacy driver. If you can not live without latest xserver then get rid of the card, you should know that since 12-4 which was the last official driver (12-6 legacy was just a bonus).

Yes, some things are slower, but other things are a lot faster. This is a known issue with TTM. There has been a Phoronix article about it.

Hi Marek are you sure that the problem is the TTM (in-kernel memory management) instead of the old compiler compared to the new LLVM Compiler from hd5000-hd6970 series ?
What’s the status of back-porting the LLVM Compiler from hd5000-hd6970 series to hd2000-hd4000 series cards?
In my knowledge its the compiler for VLIW what makes the difference.
I have some euro/dollar/bitcoins for you if you fix the LLVM Compiler for the hd2000-4000 cards